As shown in FIG. 11, most conventional refrigeration systems for use in air conditioners for automobiles include a vapor-compression type refrigeration cycle employing a compressor 101, a condenser 102, a receiver tank 103, an expansion valve 104 and an evaporator 105. FIG. 12 illustrates a Mollier diagram showing a state of refrigerant in a refrigeration cycle in which the ordinate denotes pressure and the abscissa denotes enthalpy. In this figure, the refrigerant is in a liquid-phase state in the area located at the left side of the liquid-phase line, a vapor-liquid mixed state in the area located between the liquid-phase line and the vapor-phase line, and a gaseous-phase state in the area located at the right side of the vapor-phase line.
As shown by the solid line in this figure, the refrigerant is compressed by the compressor 101 to shift from the A point state to the B point state to thereby become high-temperature and high-pressure gaseous refrigerant, and then condensed by the condenser 102 to shift from the B point state to the point C state. The refrigerant condensed in this way is once stored in the receiver tank 103, and only the liquefied refrigerant is decompressed and expanded by the expansion valve 104 to shift from the C point state to the D point state to thereby become low-pressure and low-temperature mist-like refrigerant. Then, this refrigerant is evaporated and vaporized by exchanging heat with the ambient air in the evaporator 105 to shift from the D point state to the A point state, and turns into gaseous refrigerant. Here, the enthalpy difference from the D point state to the A point state is equivalent to the quantity of heat which acts on the air-cooling. Therefore, the larger the enthalpy difference is, the larger the refrigerating capacity becomes.
By the way, in order to enhance the refrigerating capacity in the aforementioned refrigeration cycle, a condenser has been developing based on the concept that the enthalpy difference at the time of evaporation is increased by subcooling the condensed refrigerant to the temperature lower than the temperature at the C point state by several degrees to increase the amount of heat rejection at the condensing process in which the refrigerant shifts from the B point state to the C point state.
As one of such improving techniques, a condenser with a receiver tank in which the receiver tank is placed between the condensing portion and the subcooling portion has been proposed.
As shown in FIG. 13, this proposed condenser with a receiver tank is called a subcool system condenser or the like. The condenser is provided with a multi-flow type heat-exchanger core 111 and a receiver tank 113 attached to one of the headers 112. The upstream side of the heat-exchanger core 111 constitutes a condensing portion 111C, and the downstream side thereof constitutes a subcooling portion 111S independent to the condensing portion 111C. In this condenser, the refrigerant introduced via the refrigerant inlet 111a is condensed by exchanging heat with the ambient air when the refrigerant passes through the condensing portion 111C, and the condensed refrigerant is introduced into the receiver tank 113 to be separated into a liquefied refrigerant and a gaseous refrigerant. Only the liquefied refrigerant is then introduced into the subcooling portion 111S to be subcooled, and then flows out of the refrigerant outlet 111b. 
In the refrigeration cycle including this condenser, as shown by the broken line in FIG. 12, the refrigerant compressed by the compressor shifts from the A point state to the Bs point state to become high-temperature and high-pressure gaseous refrigerant, and then is cooled by the condensing portion 111C to shift from the Bs point state to the Cs1 point state to thereby become liquefied refrigerant. Furthermore, after passing through the receiver tank 113, the liquefied refrigerant is subcooled by the subcooling portion 111S to shift from the Cs1 point state to the Cs2 point state. Then, this liquefied refrigerant is decompressed and expanded by an expansion valve to shift from the Cs2 point state to the Ds point state, and turns into mist-like refrigerant. The mist-like refrigerant is then evaporated and vaporized by an evaporator to shift from the Ds point state to the A point state, and turns into vapor refrigerant.
In this refrigeration cycle, by subcooling the condensed refrigerant as shown in Cs1−Cs2, the enthalpy difference at the time of evaporation (Ds−A) becomes larger than the enthalpy difference (D−A) at the time of evaporation in the normal refrigeration cycle. Therefore, an outstanding refrigeration effect can be obtained.
The aforementioned conventional proposed condenser with a receiver tank is mounted in a limited space of an automobile like other existing condensers, and has fundamentally the same size as that of the existing condenser. However, since the conventional proposed condenser with a receiver tank uses the lower portion of the core 111 as a subcooling portion 111S which does not contribute to condensation, as compared with the existing condenser, the condensing portion 111C becomes small by the subcooling portion 111S, and therefore the condensing capacity deteriorates. Accordingly, it is necessary to increase the refrigerant pressure by a compressor and send high-temperature and high-pressure refrigerant into a condensing portion 111C so that the refrigerant can be assuredly condensed irrespective of the low condensing capacity. Consequently, in this refrigeration cycle, the refrigerant pressure increases especially in the condensing area, and as shown by the Mollier diagram in FIG. 12, in the refrigeration cycle using the conventional proposed condenser with a receiver tank, the refrigerant pressure in the condensing and subcooling area (Bs−Cs2) is high as compared with a normal refrigeration cycle. Accordingly, the load of compressor becomes large, and therefore it is required to increase the size of the compressor and enhance the performance thereof, which in turn causes increased size and weight of the refrigeration system and expensive manufacturing cost.
Furthermore, since the receiver tank 113 is integrally attached to the core 111, the receiver tank 113 is located near the condensing portion 111C to thereby interfere with the condensing portion 111C. Thus, the effective cooling area of the condensing portion 111C will decrease. Accordingly, in order to suppress the reduction of the effective cooling area, it was required to further increase the size of the condenser.
It is one object of the present invention to solve the aforementioned prior art problems and provide a duplex-type heat exchanger capable of obtaining high refrigeration performance and reducing the size and weight without increasing the refrigerant pressure.
It is another object of the present invention to provide a refrigeration system capable of obtaining high refrigeration performance and reducing the size and weight without increasing the refrigerant pressure.